Process for improving magnesium alloys, especially in shaped forms



Patented Feb. 11, 1 936 UNITED STATES PATENT OFFICE rnocnss FORIMPROVING MAGNESIUM rumors, ESPECIALLY IN SHAPED roams Walther Schmidt,Paul Spitaler, and Werner Schultze, Bitterfeld, Germany, assignors, bymesne assignments, to Magnesium Development Corporation, a corporationof Delaware No Drawing. Application August 11, 1932, Serial No. 628,294.In Germany April 25, 1929 2 Claims. (01. 149-213) This invention relatesto processes of improving magnesium base alloys and particularly to suchprocesses when practiced in connection with magnesium base alloyscontaining substantiala temperature somewhat below that temperature atwhich the most fusible constituent of the alloy melts, and thereafterquenching the thus heated article. After quenching, it is usual tosubmit thearticle to an aging treatment which at temperatures belowabout 300 centigrade. By means of such a process there is firstestablished in the magnesium base alloy a state of equilibrium at hightemperature. By quenching the magnesium base alloy, this state ofequilibrium is retained at normal temperatures. The subsequent aging ofthe alloy causes the establishment in the alloy of a state ofequilibrium corresponding to a lower temperature; that is to say, theconditions produced at high temperature and retained by quenching arepartially destroyed by the aging treatment. The result of the aging is ahardening effect in the alloy as well as improvements in otherproperties of the metal. When processes of heatetreatment such as theone above described are applied to magnesium base alloy castings, thequenching operation sets up stresses in the castings which, under someconditions and particularly in the case of complicated castings, mayleadto the formation of cracks. Likewise, in most cases, these stressestend to relieve themselves either at once or after the castings aremachined or'in use, with the result that distortion or warping of thecasting takes place to such an'extent that in some cases the castingswill not serve the purpose for which they wereintended.

The methods of this invention comprise improving magnesium base-alloyscontaining between about 1 and about percent of alloying constituentwhich forms a solid solution with magnesium and which increases insolubility in the magnesium as the temperature of the alloy isincreased, comprising heating the alloy to above about 325.centigradebut below the temperature at which the most fusible constituent of saidalloy fuses, and thereafter cooling said alloy through the temperaturerange of 300 centigrade'to at least 100 centigrade atan average rate ofnot more than about 66 centigrade per hour.

In order to produce a state of equilibrium which is stable at hightemperatures, the castings are first subjected to a heat-treatment aboveabout 350 centigrade but below the temperature at which the most fusibleconstituent of said alloy fuses. In commercial practice it is prefer+able to heat-treat at a temperature which is but a few degrees below thetemperature at which the most fusible constituent of the alloy fuses,but heat-treatment within the range defined will produce results. Thisheat-treatment having been completed in a manner well known to the art,the alloy is then cooled to room temperature comprises heating it forextended periods of time or to normal temperatures at a rate which isconsiderably slower than that rate of cooling undergone when the alloyis allowed to cool in the open air. We have determined that it isparticularly important .to retard the cooling in the temperature rangefrom about 300 to 100 centigrade, and we have further determined that inorder to produce the most desirable results, the average rate of coolingbetween the temperatures of 300 and 100 centigrade should not be morethan about 66 centigrade per hour.

The duration of the cooling in the temperature range below 100centigrade is, according to our discoveries, of less vital importance,and retarded cooling need not be practiced throughout this range.

As a result of the practice of the methods. of our invention, thecastings produced are relieved of the stresses and strains which havehitherto been present in such castings when they were treated accordingto the prior methods above described. Moreover, this effect has beenachieved without a material loss of the beneficial properties induced byheat-treatment in magnesium base alloys of this character.

The magnesium base alloys to which this invention refers' are thosealloys containing at least about 70 per cent of magnesium and about 1 to15 per cent of alloying constituent which forms a solid solution withmagnesium and which increases in solubility in magnesium as thetemperature of the alloy is increased. Among such alloying elements arealuminum, tin, lead, manganese, zinc, etc.

v Of these alloys, magnesium base alloys containing aluminum areparticular useful commercially, and the methods of our intion inconnection with magnesium alloys con-.

vention will find immediate commercial applicataining about 4 to 14 percent of aluminum; the magnesium base alloys containing 6 to 12 per centof aluminum being the magnesium-aluminum alloys in which the effect ofour nov'el methods of treatment is most pronounced. Other commercialalloys containing tin or manganese or lead or zinc or combinations ofthese elements among'themselves or with aluminum are likewise materiallybenefited when the heat-treatments above described are practicedin'connection therewith. For instance, good results are.

obtained in the case of magnesium base alloys containing 6 to 10 percentof tin or 0.5 to 6 per cent of zinc and, in fact, in connection with anyof the magnesium base alloys in which the older methods ofheat-treatment have caused improved properties.

taining the solid solution-producing alloying elements of the classabove described may be added one or more other alloying elements such ascalcium, copper, barium, silicon, antimony, nickel, cadmium, etc.,'which impart to such alloys useful properties or modify some property ofthe alloy.

As an example of the practice of the' methods of our invention inconnection with a highly useful magnesium base alloy may be cited thefollowing:

Castings of a magnesium alloy containing about 8.2 per cent of aluminumwere heated at 410 centigrade for '72 hours and then cooled to 100centigrade-in one case during 24 hours, and in To the magnesium basealloys.con-

ment, the following values for the mechanical properties were obtained:

K2 Elonga- Contrac- Elastic Yield tion tion limit point KqJsq. KqJsq.KgJ-iq. mm. Percent Percent mm. 11m. Original conditiom 16. 6 2. 7 l. 84. 7 l0. 3 Heated and cooled during 24 hrs. 23. 3 5. 6 7. 6.0 12.0Hosted and cooled during 48 hrs 23. 6 3. 4 5.0 7. 4 l3. 9

1. A process for improving the physico-me-' chanical properties of amagnesium alloy containing about 8.2 per cent of aluminum, which processcomprises heating the'alloy at 410 C.

for 72 hours and then cooling the alloy down to 100 C. in a furnacewithin about 24 hours, the temperature'drop being so arranged that thetime allowed for traversing the temperature zone between 300" and 100 C.is at least about 18 hours. 2. A process for improving thephysico-mechanical properties of a magnesium alloy containing about 8.2per cent of aluminum, which process comprises heating the alloy at 410C.

4 for 72 hours and then cooling the alloy down to 100 C. in a furnacewithin about 48 hours, the temperature drop being so arranged that thetime allowed for traversing the temperature zone between 300 and 100 C.is about 40 hours.

'WAL'I'HER SCHMIDT.

PAUL SPITALER. WERNER. SCHULTZE.

